The invention pertains to an ultrasonic measuring device that is especially suited to non-destructive testing of materials. It is fitted with an ultrasound test head, a transmitter and a receiver. A broad-band logarithmic amplifier that is built up of several amplifier stages which are identical in construction and which are connected in series is incorporated in the head.
Ultrasonic measuring devices that function according to the impulse echo process employ amplifiers that have a pitch range value for amplification of around 10.sup.5 (100 dB). In linear amplifiers the echo display on the monitor is proportional to the input voltage of the echo signal at the ultrasonic head. In a logarithmic amplifier the displayed echo height is proportional to the logarithm of the voltage in the test head. The logarithmic amplifier possesses in comparison to the linear amplifier a significantly greater dynamic range (relationship of the greatest to the smallest signal that can be detected on the monitor). Disadvantageous is however the required resolution of the maximum amplitude and the distortion of the signal type. These result from the amplified dynamic range.
Ultrasonic measuring devices of the type noted above in the introduction are familiar from the DE book by J. and H. Krautkramer: "Testing Materials by Means of Ultrasound," 4th. Ed., published by Springer. The increasing use in modern ultrasound testing systems of computer controlled amplifiers enables electronically regulated amplifiers to be employed more and more. It is advantageous if the adjustable amplifier can be switched between linear and logarithmic function. A choice can then be made between the large dynamic range that can be achieved.
In the ultrasonic measuring devices that are already familiar, the logarithmic amplifier is composed of five individual high-frequency amplifiers that are of identical construction and connected in series. For each individual amplifier stage the amplifying factor is the same, i.e., in a five-stage 100 dB amplifier the function for each is 20 dB. In addition, for all stages the saturation point at which the input (jumping-off) signal, in spite of increasing input voltage, remains constant, is also absolutely the same. By means of a video rectifier stage at the head of this chain, as well as behind each amplifier stage, a video signal is shunted off and directed to an adding stage.
In actual practice such a logarithmic amplifier is difficult to construct. Especially problematic is the requirement that all amplifier stages demonstrate the required identical qualities.
The data sheet concerning the Plessey integrated amplifier SL 531 C offers another way to construct a logarithmic amplifier from a series of individual, similar amplifier stages. This sheet describes a non-symmetric differential amplifier stage with two differential amplifiers. In a specific example a logarithmic amplifier is achieved by linking six such amplifiers in series. In this example, the logarithmic signal appears only at the head of the chain, and an adding stage is not required. Each of the differential amplifiers at any one amplifier has its own source for constant current, both the current sources being adjustable externally. It is not possible to regulate individually a single constant-current source in the integrated amplifier.
Even in this logarithmic amplifier two problems result from variations in the characteristics of the individual amplifier stages which are arranged in series. In actual practice it is not possible to ensure that all amplifier stages possess identical characteristics to a sufficient degree. Thus, this kind of amplifier always has a characteristic that to a greater or lesser degree detracts from the desired logarithmic function.
It is the purpose of the invention to further develop the ultrasonic measuring device so that the logarithmic amplifier can be regulated in such a way that, over a long period of time, its curve diverges as minimally as possible from a logarithmic amplifier curve.